Tar sand, also known as oil sand and bituminous sand, is now well recognized as a valuable source of hydrocarbons. There are presently two large plants producing synthetic crude from the tar sands of the Athabasca region of Alberta. In these operations, the tar sands are first mined and the bitumen is then extracted from the tar sand by a process called the hot water process. The extracted bitumen is subsequently upgraded by refinery-type processing to produce the synthetic crude.
The tar sand is a mixture of sand grains, connate water, fine minerals solids of the particle size of clay minerals, and bitumen. It is commonly believed that the connate water envelops the grains of sand, the fine solids are distributed in the water sheaths, and the bitumen is trapped in the interstitial spaces between the water-sheathed grains.
The hot water process is now well described in the patent and technical literature.
In broad summary, this process comprises first conditioning the tar sand, to make it amenable to flotation separation of the bitumen from the solids. Conditioning involves feeding mined tar sand, hot water (180.degree. F.), an alkaline process aid (usually NaOH), and steam into a rotating horizontal drum wherein the ingredients are agitated together. Typically, the amounts of reagents added are in the following proportions:
tar sand--3250 tons PA1 hot water--610 tons PA1 NaOH--4 tons (20% NaOH) PA1 (1) some process aid was needed for good primary recovery; PA1 (2) the process aid functioned by generating surfactants within the slurry, which surfactants were required to maximize bitumen recovery; and PA1 (3) different tar sand types, having different fines contents, would require different quantities of NaOH in order to achieve maximum primary froth production. PA1 (1) determining, for the extraction apparatus or circuit used, a measure of C.sub.o for one tar sand feed; PA1 (2) then establishing C.sub.s from time to time as different tar sand feeds are processed; and PA1 (3) varying the process aid addition to the process as the nature of the tar sand feed changes, to thereby maintain C.sub.s as close to C.sub.o as possible. PA1 (1) C.sub.s is found to equate substantially with C.sub.o ; and PA1 (2) the primary froth recovery obtained is greater than that which would be obtained if one processed each of the tar sand feeds separately.
Enough steam is added to ensure an exit temperature of the mixture from the drum of about 180.degree. F. The residence time in the drum is typically about 4 minutes.
During conditioning, the mined tar sand (in which the bitumen, connate water and solids are tightly bound together) becomes an aqueous slurry of porridge-like consistency, wherein the components are in loose association.
The slurry leaving the drum is screened, to remove oversize material, and then flooded or diluted with additional hot water. The product typically comprises 7% by weight bitumen, 43% water and 50% solids. Its temperature is typically 160.degree.-180.degree. F.
The diluted slurry then is transferred to the primary separation operation, where it is introduced into a large separation vessel having a cylindrical upper section and conical lower section. Here the slurry is retained for about 45 minutes in a quiescent condition. Most of the sand sinks to the bottom and is discharged, together with some fines, water, and bitumen, through an outlet. This discharge is discarded as tailings.
The bitumen present in the separation vessel exists in the form of globules, some of which attach themselves to air bubbles entrained in the slurry during conditioning. The aerated bitumen tends to rise through the slurry and is recovered as a froth by a launder extending around the upper lip of the separation vessel. This froth is called primary froth. Typically, it comprises:
______________________________________ 66.4% bitumen 8.9% solids 24.7% water. ______________________________________
Not all of the bitumen becomes sufficiently aerated to rise into the primary froth product. Much of this bitumen, together with much of the fines, tends to collect in the mid-section of the separation vessel. This aqueous mixture is termed "middlings".
The middlings are withdrawn from the vessel and are fed into subaerated flotation cells where secondary separation is practiced. Here the middlings are subjected to vigorous agitation and aeration. Bitumen froth, termed "secondary froth", is produced. Typically, this froth comprises:
______________________________________ 23.8% bitumen 17.5% solids 58.7% water. ______________________________________
It will be noted that the secondary froth is considerably more contaminated with water and solids than the primary froth. One seeks to minimize this contamination, as the froth stream requires downstream treatment to remove solids and water, before it can be fed to the upgrading process.
It is desirable to operate the process so that as much of the bitumen as possible reports to the primary froth. The efficiency with which bitumen is collected as primary froth is a measure of the success with which the entire bitumen in the tar sand feed has been brought to a condition amenable for spontaneous flotation. For this reason, one may consider maximizing primary recovery as optimizing the entire process.
Now, the tar sand feed to the hot water process is not uniform in nature. Its properties vary in accordance with factors such as bitumen content, fines content, nature of the coarse solids, extent of ageing and weathering after mining, and the chemical nature of the bitumen. This variation in properties of the feedstock requires that the processing conditions be altered from time to time with a view to maximizing primary froth recovery. Some optimizing techniques, such as regulating middlings density within a preferred range or maintaining the temperature within a preferred narrow range, can assist in improving recovery over a narrow variation in the nature of the tar sand feed. But there is a need for identification of a parameter which can be monitored and used to maximize primary froth recovery over a wide range of different tar sand types.
At this point, it is useful to review the role of the "process aid", as it was understood in the past. The originator of the hot water process, Dr. Karl Clark, noted that the tar sand was acidic in nature. He taught the need to add an alkaline process aid, such as NaOH, to adjust the pH of the drum slurry to near neutral condition, in order to improve bitumen recovery in the primary separation step. Later investigators taught that it was desirable to maintain a slurry pH in the range of about 8-9, to maximize bitumen recovery.
More recently, Dr. Emerson Sanford, co-worker of the present applicants, set forth in Canadian Pat. Nos. 1,100,074 and 1,094,003 that the role of the NaOH was to produce surfactants in the slurry by reaction with carboxylic and sulfonic acid substituents present in the bitumen. He submitted that it was surfactants that were needed to condition the tar sand to free the bitumen from the other tar sand components and render said bitumen amenable to air attachment. He further taught that the level of fines would affect the surfactant requirements. It was believed the fines would absorb surfactants, thereby diminishing their availability for `conditioning`. In summary, he taught that:
As mining and geological inspection of the Athabasca deposit has expanded, it has become evident that there are oil sand types that do not follow the relationships between recovery and process aid addition that one would have anticipated.
One such deviation arises from the nature of the clays. It now appears that clays differ in their capacity to adsorb surfactants. Those that are so placed in the deposit as to be in contact with bitumen can have surfaces thoroughly impregnated with hydrocarbon molecules and may not have sites available for surfactant adsorption. Clays laid down in later eras, and forming part of the overburden, may have hydrocarbon-free surfaces, in which case they are strong surfactant adsorbers. If, during mining, some of the overburden gets included in the feed sent to extraction, these non-impregnated clays "poison" the slurry by adsorbing surfactants. When the extraction circuit has been optimized for non-adsobring clays, the introduction of feed containing overburden clays will lead to reduced recovery. Some oil sands are so rich in surfactant-adsorbing clays that the power of the contained bitumen to contribute surfactants to the slurry is more than offset by the tendency of the clays to adsorb surfactants.
A second deviation from "normal" behaviour is the deterioration of oil sand after mining. During storage, feed can age. The mechanism is not understood, but the bitumen surface properties appear to alter, with the result that separation from the solids and attachment of air are made more difficult.
A third deviation arises in the case of feeds rich in bitumen. Some have been found to produce such a high level of surfactants, without any process aid addition, that the slurry is always in an over-conditioned state. Over-conditioning results in bitumen losses from the separation vessel, presumably due to emulsification.
Trying to control the process by monitoring some property of the feed is thus liable to failure because the relationships between such property and recovery can be subject to abberrations.
A safer procedure is to use some property of the slurry, once prepared, rather than of the feed, to determine the needed dosage of process aid.
There is thus a need to identify a reliable parameter which can be used to optimize NaOH addition and to determine a strategy for best combining the various types of tar sand to offset their undesirable qualities with respect to surfactant production and consumption.